Field of the Invention
The present invention relates to a sheet processing apparatus performing such processes of aligning and binding a plurality of sheets and to an image forming apparatus employing the same.
Description of the Related Art
A sheet processing apparatus requires a certain processing time in general in performing a process including a binding process or the like on sheets. This processing time exceeds, in most cases, a sheet discharge interval of the sheets discharged out of an image forming apparatus, and it is difficult to complete the binding process or the like within the sheet discharge interval.
Hitherto, in order not to drop productivity of an image forming process by interrupting the image forming process, a method of retaining the first several sheets after the beginning of the process and making them temporarily stand by during the process is widely adopted as ‘buffering’. Japanese Unexamined Patent Application Publication No. 10-181988 discloses a buffer unit configured such that a plurality of sheets fed from an upstream conveyance path is sequentially wound around a roller having a large diameter while shifting leading edges thereof so as to overlap with each other. The buffer unit then discharges the plurality of wound sheets to a downstream conveyance path in response to an output signal.
In terms of an amount of the shift among the sheets during the buffering operation, it is difficult to align the both sheets in a conveyance direction if the sheet (lower sheet) closer to a tray surface (supporting surface) of a processing tray is shifted upstream more than the sheet (upper sheet) more distant from the tray surface. Meanwhile, it is difficult to convey a plurality of sheets while precisely superposing edges of the sheets. Thus, a target value of the shift amount is set in general such that the lower sheet proceeds downstream with respect to the upper sheet. The shift amount among the sheets is set to be around 3 to 10 mm in general and to be constant during the buffering operation in the document described above and other known apparatuses.
For instance, in a case of a saddle stitching binding process (saddle processing), sheets are released onto a processing tray from a discharge roller pair. A certain sheet processing apparatus is provided with an inertia alignment system in which the sheets or the sheet bundle formed by the buffering operation are abutted against a leading edge regulating member on the processing tray and are aligned by the gravity acting on the sheet or the sheet bundle and by inertia given them in discharging operation.
Still further, there is another aligning system for aligning buffered sheets in which the lower sheet is conveyed to abut against the leading edge regulating member at first, and then the upper sheet is aligned by an alignment member such as a paddle. In the case of this aligning system, because it is necessary to align only the upper sheet, the alignment member is preferred to be a sliding member performing aligning operation while sliding on the upper side of the sheets, like the paddle, rather than a nipping member nipping the sheets. Therefore, it is difficult to increase force for aligning the sheet.
Next, an inter-stack time during which a processing operation is performed will be described. In a case of not performing buffering operation, the inter-stack time is determined by subtracting a conveyance time equivalent to a sheet length from a time interval from a leading edge of a preceding sheet to a leading edge of a succeeding sheet (referred to as Top-to-Top hereinafter) which is determined by productivity of each system. That is, the inter-stack time is expressed by Top-to-Top−L/V, where L is a sheet conveyance length and V is a sheet conveyance speed. In a case of performing the buffering operation, since the preceding sheet is retained to overlap with the succeeding sheet, the inter-stack time may be expressed as 2×Top-to-Top−L/V. Thus, it can be seen that the inter-stack time increases. Because the sheet length in the case of performing the buffering operation is a length of the overlapping sheets (a length of the sheet bundle), sheet conveyance length equals the sum of the original sheet length L and the buffering shift amount. That is, the larger the buffering shift amount, the shorter the inter-stack time is. Since this is not preferable for the original purpose of assuring a processing time, the buffering shift amount has been preferred to be as small as possible.
As illustrated in FIG. 15A, when sheets S1 and S2 are conveyed while overlapping with each other (buffering conveyance), a pasting force Fa is generated between the sheets. When sheets with a fine surface and having a high smoothness such as a coated sheet, a film, and an OHP sheet are buffered, air between the sheets is eliminated by a force bringing the sheets close to each other such as a nip pressure of a conveying roller pair for example. That is, the sheets are put into a vacuum-like condition by which the sheets are stuck on each other. In this case, the pasting force Fa becomes very large. In the case of the coated sheet or the like where pasting force Fa is large, pasting force Fa is overwhelmingly larger than an aligning force Fb (inertia force in the inertia alignment system for example) of the inertia alignment system or the alignment system employing the slide returning member. Therefore, alignment accuracy is partially lost as illustrated in FIG. 15B even after performing the alignment operation, leading to degradation of quality of resultant products.
It is desirable to perform the buffering operation from aspects of avoiding drop of productivity and of assuring the inter-stack time. However, it has been difficult to align the sheets as described above in buffering the smooth sheets such as the coated sheet, the film, and the OHP sheet. Still further, even in a case of a plain sheet whose surface is not treated, it is conceivable that pasting force Fa between the sheets increases to drop the alignment accuracy when environmental humidity is high or concentration of an output image is high (in a case of a solid coated image for example). Then, many apparatuses have been configured not to perform the buffering operation and while accepting the drop of productivity by pausing the image forming process during the processing operation.